The present invention relates to implantable medical devices, and in particular to metallic porous structures for implantable medical devices including orthopedic implants, especially acetabular cup implants, methods of forming such metallic porous layers for implants, and methods of producing such devices.
The use of orthopedic implants is the result of deterioration of human bone structure, usually because of various degenerative diseases, such as osteoarthritis. In recent years, a variety of implantable orthopedic devices have been developed. Typically, the failed bone structure is replaced with an orthopedic implant that mimics, as closely as possible, the structure of the natural bone and performs its functions.
Orthopedic implants are constructed from materials that are stable in biological environments and withstand physical stress with minimal or controlled deformation. Such materials must possess strength, resistance to corrosion, biocompatibility, and good wear properties. Also, the implants include various interacting parts, which undergo repeated long-term physical stress inside the body.
A breakdown of a permanently installed implant leads to pain, limitation on the range of motion, and may require a replacement of the implant. For these reasons, among others, the bone/implant interface and the connection between various parts of the implant must be resistant to breakdown. It is especially important since installation of an orthopedic implant often involves an extensive and difficult medical procedure, and therefore replacement of the installed implant is highly undesirable.
The requirements for the useful life of the implant continue to grow with the increase in human life expectancy. The strength and longevity of implants in large part depend on. the bone/implant interface. Various methods of connection are known in the art. For example, a hip joint is a ball-in-socket joint, and includes a rounded femoral head and a cup-like socket (acetabular cup) located in the pelvis. The surfaces of the rounded femoral head and the acetabular cup continually abrade each other as a person walks. The abrasion, along with normal loading, creates stress on the hip joint and adjacent bones. If the femoral head or the acetabular cup is replaced with an implant, this stress must be well tolerated by the implant's bearing surfaces to prevent implant failure.
FIG. 1 shows a typical hip replacement system that includes an acetabular cup prosthetic implant assembly 10 and a femoral prosthesis 20. Generally, the acetabular cup implant 10 includes a bone interface shell 11 and a socket bearing insert 12. The femoral prosthesis 20 includes a femoral stem 21 and a femoral head in the form of a ball 22, which moves inside the socket insert 12 of the acetabular cup implant 10. The femoral ball 22 usually has a polished surface to maintain a low friction interface with the surface of the socket insert 12 of the acetabular cup implant 10. The stem section 26 is inserted into the interior of the femur.
The socket insert 12 is usually made from a plastic material such as polyethylene or ultra high molecular weight polyethylene (UHMWPE), but may be of any biocompatible bearing material that has sufficient strength and wear resistance to withstand the loading and the abrasive nature of the joint. The socket insert 12 is typically held in the shell 11 by a series of locking grooves or notches. Ceramics and metals are also used to make socket insert 12 and in some instances the socket insert 12 is integral with the bone interface shell 11 so that the interior surface of the shell 11 acts as the bearing. The shell 11 is typically made from a metal such as titanium or cobalt-chrome alloy, and has a bone interface surface 16.
In use, the complete acetabular cup implant 10 may be attached to the patient's pelvis by a series of locking grooves, pins or screws 29, usually in conjunction with bone cement. Alternatively, the acetabular cup implant 10 may be press-fit by being driven into the patient's acetabulum with an impaction tool in situations where patient-related criteria are met. Press fit cementless acetabular cups have become the gold standard for an acetabular bearing component due to the favorable clinical results and surgical advantages. The most desirable cementless acetabular cup is a cup with a three dimensional bony in-growth microstructure surface, such as a porous surface layer. This method avoids the use of bone cement. The bone ingrowth into the voids of the porous bone interface layer provides skeletal fixation for the implants used for replacement of bone segments. In addition to providing a strong fixation, the bone ingrowth improves biocompatibility of the implant and is even believed by some to promote positive biochemical changes in the diseased bone. To implement this approach, it is important to develop improved methods of constructing porous outer layers on the bone interface surfaces of implants.
Depending on the type of bone, the location of the bone within the body, individual characteristics and disease state, bone has a wide variation in mechanical characteristics. Bone is generally categorized as trabecular or cancellous bone, which is porous and has an open cancellated structure, and cortical bone, which is dense. Subchondral bone, as found in the acetabulum, is also dense. It should be noted that the mechanical property values for trabecular and cortical bone overlap and often form a continuum within a given bone of the body. Wide ranges of mechanical properties for bone are reported in the literature, even for bone of the same type. Typically, trabecular bone has a tensile strength of 8 MPA as a mid range value. Cortical bone has a tensile strength approximately 15 times higher, 120 MPA as a mid range value. For comparison, the common implant metal titanium Ti6-Al4-V alloy, in solid form, has a tensile strength of 965 MPA. Cortical bone has a modulus of elasticity on the order of 30 times higher than the apparent modulus of elasticity of trabecular bone.
Orthopedic implants with porous bone interface surfaces have been studied extensively over the last twenty years. In view of the strength and longevity requirements, the implants are typically made of biocompatible metals, such as titanium or cobalt-chrome alloy. It has long been known that success in facilitating the ingrowth is related to the pore characteristics of the bone interface surfaces, such as pore size, pore topography and porosity. For example, it is known that the bone ingrowth may be almost entirely non-existent if the porous layer has pore sizes of less than 10 μm, and that pore sizes greater that 100 μm facilitate the ingrowth. For further example, it is known that the pores must interconnect to allow sufficient depth of bone ingrowth and flow of biological fluids.
Thus, one of the challenges is to provide metallic orthopedic implants having porous metallic bone interfaces with appropriate pore sizes, high pore connectivity and high porosity. Another challenge is to provide appropriate matching of mechanical properties between the porous layer and the underlying solid substrate, such as the inner solid portion of the shell 14 and the porous portions of the shell 11, respectively, of the acetabular cup implant 10 shown in FIG. 1. Likewise, it is desirable to match the properties of the porous outer portion of the shell 13 with the characteristics of the bone, which change over the surface of the implant. For example, the equatorial rim 17 of the acetabular cup implant 10 contacts dense bone in the corresponding region of the acetabulum while the dome region near the pole 18 contacts more porous cancellous or less dense subchondral bone in the corresponding region of the acetabulum. Additionally, it is desirable to match the mechanical properties of the implant to the bone in order to minimize the differences in the strain field at the interface to avoid loosening the bone/implant bonds and to properly distribute loading over the bone so as to avoid stress shielding and consequent resorption of bone. This requires matching the modulus of elasticity of the outer layer of the porous metal coating with the bone modulus of elasticity over any portion of the bone/implant interface subject to load. On a macro level, it is further desirable to match the overall resilience of the dome region of the acetabular cup implant to the resilience of the acetabulum to provide a more natural dynamic load path into the pelvis and better absorb impacts on the joint.
An additional challenge is that implants with an open porous structure are generally mechanically weaker than those with a denser, less porous, structure. It is desirable to improve the integrity of porous structures in highly stressed regions during installation and during use, both from a mechanical and a biological perspective.
Considering the conflicting requirements for optimizing tissue ingrowth, substrate compatibility, bone compatibility and mechanical considerations for the porous bone interface surfaces particular to a given implant, there is a need for implant designs and manufacturing methods that better meet these conflicting requirements.
Certain orthopedic implants having porous bone interface surfaces, and related methods of making such implants have been patented. Porous surfaces created by plasma spraying, flame spraying or sintering of metal particles such as spheres or wires onto the implant substrate are well known in the art. Such methods do not provide a high porosity (typically they are below 50%), high cell interconnectivity or optimum control of pore characteristics. For example, U.S. Pat. No. 5,926,685 describes a method of forming an implant having a porous outer surface by using an organic binder compound to enhance the binding between the porous surface layer and the implant. The binder and metal particles that form the porous layer are mixed and the mixture is placed in contact with a solid surface of the metallic implant. Then, the particles (pre-cursor of the porous layer) are bound to each other and to the solid surface of the implant via a sintering process.
In general, methods of producing high porosity metallic structures with controlled pore characteristics are known in the art. As shown in FIG. 2, such structures are typically cancellated space frame structures with struts or webs 32 defining somewhat regular shaped pores 34 and have high interconnectivity and relatively uniform pore characteristics. Typically this type of structure is formed based on the shape of a polymer foam precursor, either used as a skeleton for metallic coating or used to create a mold for a casting. The porosity of such structures is typically 60%-90% or even higher. This type of structure is very similar to that of cancellous bone, where the bone trabeculae form the struts. Furthermore, such a cancellated space frame structure is highly interconnected, allowing, in the case of a structure suitable for an implant, bone growth, vascularization and fluid flow throughout the interconnected pores.
U.S. Pat. No. 5,282,861 describes such an open cell structure for bone implants having pore volume of from 70 to 80%. The open cell structures of the '861 patent are formed by chemical vapor deposition of tantalum on a carbon skeleton to form a carbon-tantalum composite. The resulting structures have a carbon core and a tantalum outer surface. The specification of the '861 patent emphasizes that the three-dimensional porosity of this structure is uniform and consistent.
U.S. Pat. No. 5,976,454 describes a process for producing nickel foam for use in making battery electrodes. The porosity of the foam is over 90%, but it is produced by a method that is in many respects not suitable for producing foams of biocompatible metals typically used in making implants, such as tantalum or titanium.
In order to partially address the conflicting requirements for optimizing tissue ingrowth, substrate compatibility, bone compatibility and mechanical considerations for the porous bone interface surfaces particular to a given implant, several patents have proposed limited variations in the directional pore characteristics of the bone interface surfaces. U.S. Pat. No. 4,542,539 describes an implant with multiple layers of particles deposited on the surface. As shown in FIG. 3, the particles 42 increase in size from the substrate 44 of the implant to the bone interface surface 46 in the direction normal to the substrate as shown in FIG. 3. Purportedly, variation in the angle of incidence of the particles in the layer by layer deposition of the particles by a flame-plasma process results in a larger pore size and porosity at the surface. The porosity and interconnectedness is not said to be directly controllable. Because the particles occupy the bulk of the volume, this type of porous structure cannot approach the porosity, interconnectivity and mechanical integrity of a cancellated structure. The reference distinguishes variation of the pore sizes in the direction normal to the substrate from other references that have different pore sizes on different parts of the surface such as U.S. Pat. No. 5,489,306 discussed below.
U.S. Patent Application No. 2005/0100578 describes a method of achieving a porosity gradient in a porous implant through the thickness of a material by stacking layers of porous sheets and bonding the sheets together. Purportedly, the sheets can be fabricated so that features align from sheet to sheet to create a controlled porosity gradient in the direction normal to stacking.
U.S. Pat. No. 5,489,306 describes an implant with graduated size particles in different zones of the surface from the proximal to distal end of the implant to create different pore sizes on different zones of the surface. Each zone only has pores within a designated, relatively narrow, range, with the goal of encouraging a specific extent and type of osteointegration in each zone. No variation in the pore size in the direction normal to the substrate is disclosed and there is no discussion of the variation of other pore characteristics such as the porosity.
U.S. Pat. No. 6,913,623 describes an implant with a metal core and a proximal body fused to the core having a lower modulus of elasticity and a higher porosity than the core. The porosity of the proximal body may be variable or functionally gradient throughout. There is no disclosure of how the porosity varies or how the variation is to be achieved.
U.S. Pat. No. 5,986,169 describes implants formed from Nickel-titanium alloys that are made porous by a combustion synthesis method developed in Russia. While claiming to produce a controlled porosity and porosity distributions, the porosity distributions described are broader than those described in other references for optimization of a porous implant. No description of gradient porosities or how to achieve a gradient porosity is provided.
U.S. Patent Application No. 2003/0074081 purports to describe a method of producing implants having a controlled and directional gradient of porosity through all or one or more portions of an implant. Allusions are made to combustion synthesis methods generally as providing the method of creating a graded porosity. The only related example provided in the application describes the Nickel-Titanium combustion synthesis method developed in Russia that is the topic of U.S. Pat. No. 5,986,169 described above. The example provided does not disclose a method for creating a gradient porosity implant.
Various patents describe acetabular cups with resilient and relatively flexible dome regions for the purposes of reducing impacts to the bearing surfaces and the implant/bone interfaces. This has the advantages of potentially increasing the bearing life, preserving the fixation of the cup, and increasing the comfort of implant to the user. Examples include U.S. Pat. No. 6,136,033, and United Kingdom Patents 2,126,096, 1,189,325 and 1,527,498. These devices allow relative movement of the bearing surface with respect to the cup outer surface contacting the bone by allowing the outer surface to freely flex or by interposing an elastomeric material between the bearing surface and the outer surface. There is a need to provide increased resilience and decreased rigidity of the dome region of an acetabular cup without the additional cup thickness, machining and components required for these methods.
It is also desirable to create an acetabular cup with a relatively more rigid rim region of the cup. Because the cup is initially an interference fit in the acetabular socket, the cup may have large radial loads in the rim region once installed that can distort the hemisphere in that region inward and create excess bearing stress and even binding in the equatorial region of the bearing surface. This problem can be particularly aggravated by an under-reamed acetabular socket.
Therefore, there exists a continuing need for implantable medical devices, especially orthopedic implants, having metallic porous surfaces, blocks, layers or other porous structures for interfacing with bones and/or other tissue, with the porous structures having a variety of directionally controlled pore characteristics, including controlled porosity, controlled pore size and controlled pore size distribution, and with the porous structures having such controlled pore characteristics, strength, elasticity and configurations to optimize the performance of implantable medical devices.